The present invention, in some embodiments thereof, relates to satellite communication and, more particularly, but not exclusively to the use of small aperture terminals (e.g., terminals with very small or ultra small dish antennas, i.e., less than 1.2 meters or half meter, respectively), therewith.
Communication satellites typically handle communication traffic from and to hubs (fixed ground stations, also known as earth stations, with relatively large antennas and powerful transmission equipment) as well as from and to terminals (devices, sometimes mobile, with relatively small antennas and less powerful transmission power).
Transmission from the hub to the satellite and from there to the terminals is called “Forward transmission” while transmission from the terminals to the satellite and from there to the hub is called “Return transmission”. The channels used for each are called “Forward Channel” and “Return Channel,” respectively.
The terms “channel” and “sub-channel” are used in this disclosure to refer to one or more frequencies, for example, a band of frequencies used for transmission and reception. For the purposes of this disclosure “channel” and “sub-channel” are used interchangeably, though sub-channel is generally used in relation to a channel which is a superset of the frequencies of the sub-channel. Also, the verbs “broadcast” and “transmit” are used interchangeably to refer to the emitting of a signal by a hub or terminal and the nouns “broadcast” and “transmission” are used interchangeably to refer to the signal emitted.
The closest spacing between satellites is constrained by technical and regulatory considerations. At the present time, the separation between adjacent spots is 2° or 3° along the geostationary orbit. Multiple satellites may be present in any given spot, but only one satellite in a given spot will operate with a given frequency. As used in this disclosure, Adjacent Satellite refers to a satellite that is located in an adjacent spot along the geostationary orbit next to the Target or given satellite and is operating with the same frequency band as the Target or given satellite.
The size of an antenna, as used in this disclosure, relates specifically to the effective dimension of the part of the antenna substantially aligned with the separation between two satellites and which determines the extent to which a signal emitted from the antenna impinges on an area of space beyond the target satellite, e.g., a satellite in an adjacent spot. An antenna may be substantially circular, in which case the determining size is the diameter, or substantially rectangular or oval or square or any other shape, in which case the size is the relevant dimension.
Small antennas, by their nature, cannot be directed accurately at the desired satellite. As a result, transmissions from a small-aperture terminal in the Return Channel may potentially interfere with transmissions of a neighboring satellite's corresponding channel. To prevent this, it is believed in the art that measures must be taken to ensure that the off-axis EIRP spectral density for co-polarized signals is low enough that interference does not occur or is at acceptable levels.
In particular, in the USA, current regulatory requirements (as of drafting) require non-interference to a theoretical limit of 2° satellite separation as specified in Code of Federal Regulations, Title 47, Part 25, regulations 132 & 209 (in short: 47 CFR part 25). Other examples of regulatory requirements are the EU regulations by ETSI: Standard EN 302 186, and the international regulations by ITU-R (a UN organization): Recommendation document M. 1643 (in short: Recommendation M.1643 of the ITU-R).
Current state of the art teaches the use of spread spectrum techniques to meet the requirement for non-interference with neighboring satellites. Spread spectrum techniques as taught in the current art result in a 75% to 90% loss of bandwidth utilization. Alternative techniques, also with-a similar losses are deep coding techniques, requiring, for example, 4 bits of channel to encode 1 bit of data.
When communicating with satellites a satellite modem is used. As used in this disclosure, a “standard modem” refers to a common modem (e.g., one conforming to standards such as, but not limited to, DVB-S (European ETSI standard EN 300 421) or DVB-S2 (European ETSI standard EN 302 307)) without special auxiliary equipment (e.g., for spread spectrum, deep coding, etc.). In accordance with the prior art, a non-standard modem must be used for small antennas, for example a non-standard modem with spread spectrum capability or with deep-coding capability. Small antenna transmission systems and other parts of the communication systems are thus configured to use less than all the available bandwidth for data transmission.